Relativity refers to the idea that the velocity of an object can be determined only relative to the observer. For example, if a fly moves around the inside of a car at about 1 mile per hour, inside the car’s frame of reference, the fly is moving at 1 mile per hour. But if the car goes past you at 65 miles per hour, it will appear as if the fly is traveling at 66 miles per hour, not 1 mile per hour. In other words, it’s all a matter of reference, and it’s “relative” to your viewpoint.

What new ideas came out of Einstein’s (and others’) study of relativity? He showed that space and time could no longer be viewed as separate, independent entities, forming a four-dimensional continuum called space-time (also written as spacetime). It is not easy to verbally explain the intricacies of Einstein’s theory. The best way to interpret his works is with the use of formulations from certain mathematical branches, such as tensor calculus (for more about tensors, see “Mathematical Analysis”). But such complex equations are beyond the scope of this book.